iStockphoto/Thinkstock(NEW YORK) -- Tracy McGrady was one of the best basketball players in the world, a seven-time All-Star forward and a scoring machine, until he suffered a devastating knee injury in 2008.

When McGrady returned to the game after two operations on his left knee, he was a shadow of the player he had been before.

"It was tough to deal with because at one minute, you're playing so well, you know, on top of your game, the next minute, you're not even valuable no more," he said. "Where do I go from here?"

McGrady went to Germany.

The basketball star met with Dr. Peter Wehling, an orthopedics and sports medicine physician in Düsseldorf. Wehling discovered a new way to treat knee injuries, back pain and osteoarthritis with a controversial new treatment virtually unknown in the United States and not approved by the Food and Drug Administration. It's called Regenokine injections.

"It's a combination of proteins that attacks, in a very specific way, inflammation and pain and destruction of tissue," Wehling said.

Regenokine's key ingredient is the patient's own blood. Wehling explained that he uses a machine to extract the body's natural anti-inflammatory proteins called "InteRleukin 1-Receptor Antagonist" or "IL1-RA" proteins. Those proteins, which essentially block the inflammation receptors, are then re-injected into the injury site.

"The re-injecting [of proteins] causes a stop of inflammation," Wehling said. "You have an improvement in function and a significant decrease in pain."

Wehling said Regenokine injections are effective for two to four years, and sometimes longer.

McGrady had the treatment on his bad knee in the summer 2010, and about a week after the procedure, he said he could feel the difference.

"Not only did it feel stronger, less pain," McGrady said. "Sure enough, it felt better…since the time I hurt my knee, it's the best I've ever felt."

McGrady said he played this season with the Atlanta Hawks pain-free, and he is just one of a number of pro athletes who have undergone the Regenokine treatment for various chronic pain ailments with success.

Golfer Fred Couples had the treatment after crippling back pain almost forced him to retire. Yankees third baseman Alex Rodriguez had it last December to treat a nagging knee and shoulder injury, and now the baseball star says he feels great.

And Kobe Bryant said the Regenokine treatment did wonders for his damaged knee. He is returning to Germany this summer for further treatment.

Naturally, professional athletes have a powerful incentive to seek the best medical treatment in world because they want to keep playing. Unintentionally, these athletes have become the vanguard, the guinea pigs, some would say, in testing cutting-edge and experimental therapies for chronic pain.

"I think elite athletes push us," said Dr. Allan Mishra, an orthopedic surgeon and a member of the medical faculty of Stanford University. "They're leaders. They're pioneers. Frankly, they have the desire and means to seek out novel treatments. They do drive us to a better understanding of how they work."

Of course, it isn't just pro athletes who are hobbled by bad knees or aching backs. According to the American Academy of Pain Medicine, an estimated 26 million Americans live with the agony of chronic back pain, and millions more suffer from osteoarthritis and knee injuries.

But Regenokine treatment is expensive. It starts at around $5,000 and can easily run into tens of thousands of dollars. Wehling said its success rate is estimated at around 70 percent.

But Regenokine is not without controversy and some skepticism from medical professionals in the U.S. While many American orthopedic surgeons Nightline spoke to called the treatment "promising," others said that not nearly enough clinical studies have been done on the treatment in this country to assess its effects and possible side effects.

Dr. Rick Delamarter, a spinal surgeon and vice chair of surgery at Cedars Sinai Medical Center in Los Angeles, was leery of Regenokine because there are still too many unanswered questions.

"I always look at it, regardless of the treatment, would I use this on my own mother, would I use it on my brother, my sister," Delamarter said. "And I do want to see more long-term information: Does this give effects five, 10 years down the line? How successful is it? How long does it last? Not only is it what I want to see but the FDA is going to require those things."

The FDA declined to comment on this story and their ultimate ruling on Regenokine could take years. But even without FDA approval, there are now a handful of American doctors in L.A. and New York City who are using Regenokine "off label," which is legal.

Dr. Christopher Renna, a preventive medicine specialist in Santa Monica, Calif., worked with Wehling to create the Regenokine Treatment Program, which incorporates lifestyle changes, and was the first to offer the Regenokine treatment in the United States. The two doctors also co-wrote a book called The End of Pain, about the benefits of Regenokine.

Dr. Edward Capla and Dr. Doug Schottenstein, both with New York SportsMed in New York City, began administering the treatment last year. Capla treated McGrady for back pain during this year's All-Star break in February.

Nonetheless, Tracy McGrady said Regenokine kept him on the court.

"Had I not done that procedure, there's no way I would have continued playing in the NBA," he said. "There's no way."